This invention relates to a diamond film for x-ray lithography and a method for preparing the same.
As the manufacture of advanced semiconductor devices requires finer patterning, x-ray lithography is considered to have a promising future.
A mask used in x-ray lithography has a structure as shown in FIG. 1. The mask includes a support substrate 1, a x-ray transmissive membrane 2 disposed on one surface of the substrate 1, an x-ray absorbing pattern 3 carried on the membrane 2, and a surface protective film 4 disposed on another surface of the substrate 1. The membrane 2 must have the properties of (1) high mechanical strength, (2) resistance to irradiation of high-energy beams such as high-energy electron beams and synchrotron orbital radiation (SOR), and (3) high transmittance to visible light as needed for high precision filaments.
Heretofore, BN, boron dope, Si, SiN, SiC, diamond, etc. have been proposed as the mask membrane material for x-ray lithography. Among these, diamond satisfying the above-mentioned properties (1) to (3) is regarded optimum as the mask membrane material for x-ray lithography.
The membrane must be a self-supporting film of 2 to 3 xcexcm thick in order to minimize the absorption of x-rays. In order for the membrane to be self-supporting, the film must have a tensile stress in the range of 0.0 to 4.0xc3x97109 dyn/cm2.
In general, diamond films are prepared by microwave plasma chemical vapor deposition (CVD) and hot filament CVD processes because these processes are relatively easy to form diamond films of good crystallinity over large areas. For example, the diamond film preparation method which is most commonly employed in the prior art relies on the microwave plasma CVD process using hydrogen-diluted methane as a source gas. If deposition is carried out at a low methane concentration, a diamond film of good crystallinity is obtained, but the film is always compressive stressed. Despite good crystallinity, the film cannot be self-supporting. Such a film cannot be utilized as the x-ray mask. Conversely, if deposition is carried out at a high methane concentration, a diamond film having tensile stresses produced therein is obtained at the sacrifice of crystallinity. Since the microwave plasma CVD and hot filament CVD processes utilize plasma, many film deposition parameters are correlated to plasma conditions. It is, therefore, quite difficult to form a diamond film capable of satisfying both crystallinity and film stress which are essential physical properties for diamond films to serve as the x-ray mask.
An object of the invention is to provide a method for preparing a diamond film while the stress produced during deposition is precisely controlled so as to form a tensile stressed diamond film without reducing the crystallinity thereof whereby the diamond film can serve as a membrane for x-ray lithography.
Another object of the invention is to provide a high crystallinity diamond film having tensile stresses.
A diamond film is formed on a silicon single crystal substrate by a microwave plasma CVD or hot filament CVD process using hydrogen-diluted methane as a source gas. We have found that when a gaseous mixture of methane (CH4) gas, hydrogen (H2) gas and oxygen (O2) gas is used as the source gas, and the proportions (% by volume) of the respective gases are set to fall in the ranges:
3.0%xe2x89xa6CH4xe2x89xa68.0%,
87.0%xe2x89xa6H2 less than 97.0%,
and
0.0% less than O2xe2x89xa65.0%,
there is deposited a highly crystalline diamond film exerting a tensile stress, especially a tensile stress of 4.0xc3x97109 dyn/cm2 or smaller. This diamond film is effective as a membrane for x-ray lithography. When film deposition is carried out while keeping the silicon single crystal substrate at a temperature of 900xc2x0 C. to 1,000xc2x0 C., the tensile stress can be controlled. In summary, we have found that when diamond is deposited by the microwave plasma CVD or hot filament CVD process under controlled conditions including (1) the volume proportions of methane (CH4), hydrogen (H2) and oxygen (O2) of the source gas and (2) the substrate temperature, a diamond film whose internal stress is controlled at a high precision can be produced without detracting from the crystallinity thereof.
We have further found the following. The diamond film has a hydrogen concentration [H] and an oxygen concentration [O] at a depth of 0.5 xcexcm from its surface as measured by secondary ion mass spectroscopy (SIMS). The film is more effective when the hydrogen and oxygen concentrations [H] and [O] fall in the ranges: 5.0xc3x971019 atoms/cm3xe2x89xa6[H]xe2x89xa65.0xc3x971021 atoms/cm3 and 2.0xc3x971018 atoms/cm3xe2x89xa6[O]xe2x89xa65.0xc3x971020 atoms/cm3. The diamond film exhibits a peak attributable to diamond near 1333 cmxe2x88x921 and a peak attributable to amorphous carbon at 1530 cmxe2x88x921 as analyzed by laser Raman spectroscopy using a laser beam having a wavelength of 532 nm and a diameter of 150 xcexcm. The film is more effective when the ratio of the intensity of the former peak to the intensity of the latter peak, I(1333 cmxe2x88x921)/I(1530 cmxe2x88x921), is in the range: 1.20xe2x89xa6I(1333 cmxe2x88x921)/I(1530 cmxe2x88x921)xe2x89xa61.50. The diamond film exhibits a peak attributable to (111) oriented diamond at a diffraction angle (2xcex8) of 43.9xc2x0 and a peak attributable to (220) oriented diamond at a diffraction angle (2xcex8) of 75.3xc2x0 as analyzed by x-ray diffractometry at a wavelength xcex of 1.54 xc3x85. The film is more effective when the ratio of the intensity of the former peak to the intensity of the latter peak, I(111)/I(220), is in the range: 3.0xe2x89xa6I(111)/I(220)xe2x89xa68.5.
In a first aspect, the invention provides a method for preparing a diamond film for x-ray lithography, comprising the step of depositing diamond on a silicon single crystal substrate by a microwave plasma CVD or hot filament CVD process using hydrogen-diluted methane as a source gas, characterized in that oxygen gas is added to the source gas so that the resulting source gas consists essentially of 3.0% to 8.0% by volume of methane gas, 87.0% to less than 97.0% by volume of hydrogen gas, and more than 0.0% to 5.0% by volume of oxygen gas. Preferably, the silicon single crystal substrate is heated at a temperature of 900 to 1,000xc2x0 C. while diamond is deposited thereon.
In a second aspect, the invention provides a diamond film for x-ray lithography prepared by the above method and having a tensile stress of 4.0xc3x97109 dyn/cm2 or smaller.
The invention also provides a diamond film for x-ray lithography prepared by the above method and having a hydrogen concentration [H] and an oxygen concentration [O] at a depth of 0.5 xcexcm from its surface as measured by SIMS which fall in the ranges:
5.0xc3x971019 atoms/cm3xe2x89xa6[H]xe2x89xa65.0xc3x971021 atoms/cm3 and
2.0xc3x971018 atoms/cm3xe2x89xa6[O]xe2x89xa65.0xc3x971020 atoms/cm3.
The invention further provides a diamond film for x-ray lithography prepared by the above method wherein when analyzed by laser Raman spectroscopy using a laser beam having a wavelength of 532 nm and a diameter of 150 xcexcm, the diamond film exhibits a peak attributable to diamond near 1333 cmxe2x88x921 and a peak attributable to amorphous carbon at a Raman shift of 1530 cmxe2x88x921, and the ratio of the intensity of the former peak to the intensity of the latter peak, I(1333 cmxe2x88x921)/I(1530 cmxe2x88x921), is in the range: 1.20xe2x89xa6I(1333 cmxe2x88x921)/I(1530 cmxe2x88x921)xe2x89xa61.50.
Still further, the invention provides a diamond film for x-ray lithography prepared by the above method wherein when analyzed by x-ray diffractometry at a wavelength xcex of 1.54 xc3x85, the diamond film exhibits a peak attributable to (111) oriented diamond at a diffraction angle (2xcex8) of 43.9xc2x0 and a peak attributable to (220) oriented diamond at a diffraction angle (2xcex8) of 75.3xc2x0 , and the ratio of the intensity of the former peak to the intensity of the latter peak, I(111)/I(220), is in the range: 3.0xe2x89xa6I(111)/I(220)xe2x89xa68.5.